The Time-Triggered Protocol and its use for industrial communications
The needs for industrial communication have increased especially since the 90s in regard to the specification of the fieldbus standard. Since then, it has been considered increasingly necessary to stimulate and enable the individual network participants for communication within a system. Protocols with their many subtypes covering different sectors of the industry have been used traditionally to manage such an interaction. The Time-Triggered Protocol can be found in railway and aviation engineering, but also in the automotive sector. After all, these sectors require the sending of time-critical signals.
The Clock Synchronization of the TTP Protocol
The Time-Triggered Protocol is a kind of protocol based on the use of fieldbuses and which is designed for the transfer of data. The special feature of the TTP is its fault-tolerant communication, which takes place time-controlled between the electronic components. Additionally, hard real-time requirements are realized here. The capacity for time-controlled transfer of data results from the synchronization of the clocks of the participants, as the common clock is used as a uniform signal for control. The synchronization of the clocks is done by forming a mean value resulting from the real arrival time and the expected arrival time of the message. The algorithm resulting from this operates decentralized, by which a tolerance from the Time-Triggered Protocol results in regard to the individual errors.
The Communication Division with the Time-Triggered Protocol
The arrival times expected in advance are specified in the MEDL, the Message Descriptor List. Through this list, each individual participant is informed when it may start to transmit and when its task is reception. This is then called TDMA, Time Division Multiple Access, which is a kind of division of the communication. When such a round exists in the network, then each participant has the opportunity to transfer a message during a specified time interval. When such a communication has ended, a new one can start. The transfer time of the MEDL also serves for prevention of network failures due to a defective participant. Thus it can occur that a defective participant continuously sends data for which there is no use and which does not make sense. To stop clogging of the channels, each participant in the Time-Triggered Protocol has a bus topology that serves as local bus guard, which ensures that transfer is possible only during the specified time interval. When a participant then sends a message, it must first be supplemented with a small header, which creates a frame that is then protected by a CRC checksum.
The second component of the Time-Triggered Protocol
The conventional classification of the TTP protocol, is Time-Triggered Protocol class C, but there is also a variant of class A. This time-controlled protocol serves for real-time enabled control of sensors and actuators. Furthermore, it is also designed for control units that do not have to operate time-critical. TTP/A, the class designation used in engineering, functions with a TDMA scheme that is available for collision-free message transfer. This protocol type is a master-slave system where the communication is divided into rounds and slots and where exactly one UART word of the corresponding coding is sent in each individual slot.
TTP/A can be implemented with the Universal Asynchronous Receiver Transmitter based on the standard of an 8-bit micro-controller, where the layer essentially remains exchangeable. The UART coding is done with
- one start bit
- eight data bits
- one parity bit and
- one stop bit
The Membership List of the Time-Triggered Protocol
With the Time-Triggered Protocol, the membership service is used in addition to the TDMA round, when the communication protocol is to be made compatible for safety-critical applications. Here, each node has a list where the nodes considered error-free are entered. In the case of disagreements between different lists, the node itself is considered as faulty and is set to passive mode. When a node sends a faulty frame, it is excluded immediately from the list. This service is executed again with each individual communication in order to prevent problems from errors. However, this safety measure as such also has the disadvantage that groups which consider each other error-free are created and then communicate exclusively with each other. To prevent this problem, each node additionally checks whether it is a member of the largest one of these groupings. If a negative result is found, namely a membership, this node switches to passive mode.
The Advantage of the Time-Triggered Protocol for industrial communication
Use of such a real-time-enabled protocol enables safe communication in complex industrial installations, as the individual components check each other and independently, and as in case of an error or a complete failure no other participant has to remove the faulty node, as it switches itself to passive mode. This prevention of any error sources by means of the Time-Triggered Protocol enables industrial communication so that the systems and the networks can operate efficiently and target-oriented.